stdlib: Dictionary, Set: allow querying for a mismatched type when bridged

Swift's Dictionary and Set are typed, but when bridged to NSDictionary
and NSSet they should behave accordingly, that is, allow querying for
keys of arbitrary types.

rdar://problem/23679193

stdlib/public/core/HashedCollections.swift.gyb

@@ -3275,7 +3275,9 @@ final internal class _Native${Self}StorageOwner<${TypeParametersDecl}>
   @warn_unused_result
   internal func bridgingObjectForKey(_ aKey: AnyObject)
     -> AnyObject? {
-    let nativeKey = _forceBridgeFromObjectiveC(aKey, Key.self)
+    guard let nativeKey = _conditionallyBridgeFromObjectiveC(aKey, Key.self)
+    else { return nil }
+
     let (i, found) = nativeStorage._find(
       nativeKey, startBucket: nativeStorage._bucket(nativeKey))
     if found {